Field devices serving for registering and/or influencing process variables are often applied in process automation technology as well as in manufacturing automation technology. Serving for registering process variables are measuring devices having at least one sensor and one measurement transmitter. For example, the measuring devices can be fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, or conductivity. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a section of pipeline, or the fill level in a container, can be changed.
Field devices are, in principle, all devices, which are applied near to the process and deliver or process process relevant information. Besides the earlier named measuring devices/sensors and actuators, also referred to as field devices are generally units, such as remote I/Os, gateways, linking devices and wireless adapters, which are directly connected to a fieldbus and serve for communication with superordinated units. A large number of such field devices are produced and sold by the Endress+Hauser group of companies.
In modern industrial plants, field devices are, as a rule, connected via fieldbus systems (systems such as e.g. ProfiBus®, Foundation Fieldbus®, HART®, etc.) with superordinated units. Normally, the superordinated units are control systems, or control units, such as, for example, a PLC (programmable logic controller). The superordinated units serve, among other things, for process control, process visualizing, process monitoring, as well as for start-up of the field devices. The measured values registered by the field devices, especially sensors, are transmitted via the connected bus system to one or, in given cases, also a number of superordinated unit(s). Along with that, also data transmission is required from the superordinated unit via the bus system to the field devices; this serves especially for configuring and parametering of field devices or for diagnostic purposes. In general, the field device is serviced via the bus system from the superordinated unit.
Besides hardwire data transmission between the field devices and the superordinated unit, there is also the opportunity for wireless data transmission. For implementing wireless data transmission, field devices are embodied as radio-field devices, for example. These have, as a rule, a radio unit and a power supply unit as integral components. In such case, the radio unit and the supply unit can be provided in the field device itself or in a radio module connected durably to the field device. The supply unit enables an autarkic energy supply of the field device.
Alternatively, field devices without radio units—thus the base installed in the field today—are modified into radio field devices by the coupling of a wireless adapter, which has a radio unit.
In the case of autarkic field devices, which are controlled via a microcontroller, it is usual to provide a circuit, which rapidly and reliably detects failure of the supply voltage of the supply unit. In this way, the microcontroller has the opportunity to store important parameters, before also a secondary supply unit is lost. The secondary supply unit is usually an energy storer, which can still supply the circuit components of the field device for a limited time with voltage following failure of the supply unit. Known circuit variants for the detection of a so-called “Pre-Power-Fail” include resistive voltage dividers or solutions in the form of integrated circuits, which, most often, scale the supply voltage to a voltage value detectable by the microcontroller and so make the failure measurable.
Often, microcontrollers/CPUs apply Schmitt-trigger stages on their signal inputs. If such a signal input is used, smaller fluctuations, which result, in given cases, from the scaling of the supply voltage by means of voltage dividers, can be measured only inaccurately. On top of this come, most often, also tolerances of the Schmitt trigger switching levels. More exact and therewith reliable is the early detection of a failure of the supply voltage by means of a comparator input; however, this solution is unfavorable as regards energy consumption.
Special disadvantages of the scaling of the supply voltage become evident in the case of single use battery operated devices, thus in the case of devices with limited capacity: With sinking battery voltage, also the voltage on the detection input becomes smaller. Therefore, the setting of the detection threshold must assume a lowest battery voltage. This, in turn, causes a small difference between the “OK”- and the “Bad”-(fail) signal levels. As a result thereof, this type of detection is very disturbance sensitive relative to in-coupling onto the signal lines.
The detection of a Pre-Power-Fail via a resistive voltage divider or also via conventional integrated circuits requires relatively much energy, with energy being very costly, especially in the case of single use battery operated devices. The lower ohmically the divider is designed, the higher is the lost electrical current; the higher ohmically the divider is designed, the more disturbance susceptible is the circuit as regards in-couplings.